1. Field of the Invention
This invention relates to embossing surfaces for use in the plastic, textile, paper, aluminum, steel and other industries for applying embossed designs and matte finishes to embossable surfaces such as sheets of plastic, paper, textile or metals such as aluminum or steel. More particularly, this invention relates to embossing rolls and embossing tools useful for the manufacture of diverse embossed articles, including plastic or aluminum siding for buildings and mobile homes, ceiling and floor tiles, ceiling, floor and wall coverings, imitation leather, wearing apparel, upholstery materials, automotive dashboards and door panels, book coverings, aircraft cabin wall and ceiling panels, and the like.
2. Prior Art
Embossing is a method in which an embossable surface, e.g., the surface of a thermoplastic sheet, is textured with a pattern by forcing the sheet between two rolls (or other tooling), one of which has a three-dimensional pattern on its surface which is the mirror image of the pattern desired on the thermoplastic sheet. Requirements for the embossing roll (i.e., the roll with the three dimensional pattern imposed on its surface) include (a) the ability to create a repeatable, controlled surface texture tailored to the particular product requirements, (b) adequate mating characteristics vs. the backup roll (or other tooling) which is usually rubber covered, but sometimes is metal, that has the three-dimensional mirror image of the embossing roll imposed in its surface, and (c) optimized thermal characteristics relative to the thermal characteristics of the sheet being embossed. In the polyvinyl chloride, PVC, siding industry, for example, the embossing roll is intended to create a woodgrain pattern on the vinyl siding used for housing and other building construction.
Heretofore, embossing tools in the form of a metal roll or a metal stamping tool, e.g., steel or aluminum roll or stamping tool, having a patterned embossing surface, such as a woodgrain pattern engraved in the embossing surface, were chromium electroplated to provide wear resistance. In those instances, as in the polyvinyl chloride or aluminum siding industry, where it is desirable to provide a dull, non-glossy or matte finish on the embossed (e.g. woodgrain embossed) surface, the chromium-electroplated embossing surface was blasted with grit or chemically etched to produce a rough surface by forming a multitude of tiny pits in the embossing surface. These tiny pits cause the formation of multitudinous tiny peaks over most of the embossed surface to provide a matte finish thereto and thereby disperse light hitting the embossed surface thus providing it with the non-gloss, matte finish. The resulting matte finish also is intended to hide surface imperfections in the sheet being embossed. However, the matte finish produced by chromium electroplated, grit-blasted or chemically etched embossing surfaces are relatively non-uniform and become more non-uniform as the embossing surface wears.
The conventional chromium-electroplated, grit-blasted or chemically etched embossing rolls suffer severely from lack of wear resistance and corrosion resistance particularly when contacted with thermoplastic materials, such as polyvinyl chloride, containing residual HCl or other corrosive residual chemicals. As a consequence, conventional chromium-electroplated, grit-blasted or chemically etched, embossing rolls do not last long and have to be replaced frequently. The normal life of a chrome-electroplated embossing roll used to produce vinyl siding is about 7 to about 12 months after which it must be stripped, electroplated again with chromium if the steel substrate has not been damaged and grit-blasted or chemically etched again.
Furthermore, the embossed surface heretofore produced by conventional embossing surfaces suffer from the further defect of lacking scuff-resistance. In the case of PVC siding, unsightly, shiny scuff marks are readily formed on the embossed surface when it is subjected to light to moderate impact or scraping, for example, as caused by a hammer or other object during construction, by a tennis ball hitting the embossed surface or by a rake handle sliding across the embossed surface. It is believed that this is the result of the flattening or scraping off at the contact areas of the tiny peaks formed in the surface of the PVC siding by the tiny pits in the surface of the embossing roll. Furthermore, since the chromium-electroplated embossing rolls lack sufficient wear resistance, its embossing surface smooths out relatively quickly diminishing or eliminating the pits in the embossing surface, consequently diminishing or eliminating the tiny peaks in the embossed surface resulting in increasing gloss on the embossed surface and loss of ability to hide minor surface imperfections in the sheet being embossed.
Additionally, conventional chromium-electroplated, grit blasted or chemically etched, embossing rolls produce matte finishes that differ over a relatively wide range from embossing roll to roll. As a result, it is difficult, and in some instances not possible, to produce an embossed product such as PVC siding having a consistent level of gloss when a conventional embossing roll is replaced with a fresh one or the product is produced using more than one embossing line equipped with different conventional embossing rolls. The application of PVC siding having widely different levels of gloss (or conversely widely different levels of dullness) to the same building produces an unsightly appearance and is unacceptable, particularly since a prime advantage of PVC siding is that it requires no painting or other maintenance for the lifetime of the siding.
A further defect in the use of conventional chromium-electroplated, grit-blasted or chemically etched embossing rolls in the production of woodgrain, matte-finished PVC siding or similar products is that relatively minor changes in line speed at which the PVC sheet is being embossed by the conventional embossing rolls, or relatively minor variations in the temperature of the PVC sheet or of the conventional embossing roll, causes unacceptable variations in the level of gloss (or dullness) in the final product. Extensive adjustments to the embossing pressure and coolant flow rate are then necessary whenever a change in line speed or PVC sheet temperature occurs. This results in downtime and consequent loss of productivity until the proper conditions are restored. Also, a considerable amount of PVC sheet embossed with improper gloss level until the proper conditions are restored must be discarded or reprocessed.
Embossing rolls heretofore have also been coated with grit of various sizes bonded to the surface of the embossing roll by means of an adhesive such as a synthetic plastic adhesive. This prior technique is disclosed in U.S. Pat. Nos. 3,387,351, 3,539,671, 3,619,446 and 4,250,135. The problems presented by the grit-applying techniques are that the grit is usually non-uniformly applied and can result in the alteration of the embossing pattern including the filling in by the grit and adhesive of cavities, crevices and other fine detail in the embossing surface. There is also the danger that through wear and improper application the grit will flake off or otherwise become embedded in the embossed product.